As industry tends toward precision, speed, and mass production, there is a need for improving the cutting performance and lifetime of cutting tools. In particular, during high-speed cutting of a workpiece having a high hardness or when cutting a difficult-to-cut material having a low thermal conductivity, high heat of at least 900° C. is generated locally at an end of an insert which is under friction with the workpiece. By forming a hard coating film having superb oxidation resistance and wear resistance on a cutting surface of the insert, the lifetime of the cutting tool may be extended.
Typically, a single-layered hard coating film, such as TiN, Al2O3, TiAlN, AlTiN, or AlCrN, having wear resistance, oxidation resistance, or shock resistance, and the like, or a multilayered hard coating film in which at least two of such layers are laminated were formed on a base material of a cemented carbide, a cermet, an end mill, or a drill, and the like, to address the requirements related to workpieces having a high hardness or difficult-to-cut materials.
However, the hardness of workpieces is increasing and demand for processing difficult-to-cut materials having a low thermal conductivity and a severe tendency to become fused to tools is also increasing, and thus it is becoming increasingly difficult to respond to such demands merely through the development of thin film compositions having novel physical properties or through the use of simple multilayered structures.
Accordingly, attempts to improve the cutting performance through a method in which two or more nano-level thin films having different physical properties are regularly and repeatedly laminated have increased recently.
For example, Korean Patent No. 876366 discloses a thin film structure in which the erosion resistance and oxidation resistance of the top layer are improved through a structure formed by depositing an underlayer on inserts, end mills, drills, or cermet tools, which are cemented carbide tools, using physical vapor deposition (PVD) to improve the adhesiveness and for crystal orientation in a (200) plane, and successively depositing a (Ti,Al)N multilayered thin film, which is an intermediate layer and then a top layer composed of an A layer, a B layer, a C layer, and a D layer, which are composed of TiAlN or AlTiSiN and have different compositions from each other.
Through such a multilayered structure, the wear resistance and oxidation resistance may be improved, but development of a hard coating film having a novel structure is needed to improve the various properties such as wear resistance, shock resistance (toughness), and chipping resistance which are needed for the cutting process.
For this, the present inventors, as disclosed in Korean Patent No. 1284766, have disclosed a hard coating film which is formed on the surface of a base material, the hard coating film for cutting tools being characterized by having a structure in which a first layer and a second layer are alternately laminated, the first layer being composed of Al1-xCrxN (0.3≦x≦0.7), the second layer consisting of a nanoscale multilayered structure or a structure in which the nanoscale multilayered structure is repeatedly laminated at least two times, the nanoscale multilayered structure including a thin layer A, a thin layer B, a thin layer C, and a thin layer D having thicknesses of 3 nm to 20 nm, the thin layer A being composed of Al1-a-bTiaSibN (0.3≦a≦0.7, 0≦b≦0.1), the thin layer B and the thin layer D being composed of Ti1-xAlxN (0.3≦x≦0.7), the thin layer C being composed of Al1-zCrzN (0.3≦z≦0.7), and the aluminum (Al) content in the thin layer A differing from the aluminum (Al) content in the thin layer B.
Through the above described structure, the hard coating film having improved toughness (shock resistance), chipping resistance, and lubricity was obtained, but a hard coating film that is further improved in terms of wear resistance is needed.